STRUCTURE OF TWENTY-FOUR HOUR CHIOKS 



45 



establishes the neural groove takes place first in the cephalic 

 region of the embryo. At its cephalic end the neural groove 

 is therefore deeper and the neural folds are correspondingly 

 more prominent than they are caudally. The folding has not, 

 at this stage, been carried much beyond the cephalic half of 

 the embryo. Consequently as the neural folds are followed 

 caudad they diverge slightly from each other, and become less 

 and less distinct. 



ectoderm of he=>d 



border of fore-gut 



subcephalic pocket 



margin of ant. 

 intestinal portal 



area pellucida 



border of mesoderm 



sculosa 



ffelii- P 



PIG. 15. Dorsal view ( X 14) of entire chick embryo having 4 pairs of meso- 

 dermic somites (about 24 hours incubation). 



Study of transverse sections of an embryo of this stage affords 

 a clearer interpretation of the conditions in neural groove for- 

 mation than the study of entire embryos. A section passing 

 through the head region (Fig. 17, A) shows the neural plate 

 folded so it forms a nearly complete tube. Dorsally the mar- 

 gins of the neural folds of either side have approached each 

 other and lie almost in contact. The formation of the neural 

 folds takes place first in about the center of the head region, 

 and progresses thence cephalad and caudad. By following 

 caudad the sections of a transverse series, the margins of the 



